THE
CORE of the Greek tense system consists of the
Present,
Aorist
and
Present Perfect
tenses. These tenses have more or less different stems, out of which all other
tenses are formed (Imperfect
uses the stem of the
Present,
Future
uses the stem of the
Aorist,
Pluperfect
and
Future Perfect
use the theme of the
Present Perfect).
That means,
if you know the core of a verb, these three tenses, it's easy to
find everything else.

In the course of time, as you become more familiar with the various verb
forms, you will find it very easy to recognise the tenses. For the moment
we need to say just a word about the meaning of tenses, how they define
time in speech.

We say that
Present
refers to an action that happens now. In Greek it is called
Ἐνεστώς
= the time that has just now established/installed an action. Imagine
this in space terms. When drawing a line, each point, one by one is
established in and by time, and becomes instantly past. That very in-stant
in which each point appears is the
ἐν‑εστώς.
In the sentence attributed to Heraclitus
πάντα χωρεῖ καὶ οὐδὲν μένει
(=everything moves and nothing remains), a sentence describing a principle,
what gives the sense of a continuous present is just this, the sense of a
principle, and not the meaning, of course. The principle as such is always
present. We may call this a meta-present, a present that does not
belong to a grammatical event, but to a detached observation of language and
reality, as if the one who speaks was not governed by language or
reality. It is interesting, that the very word
ἐνεστώς
is a present perfect participle, the very present is described by a
present perfect word! We would be closer to reality if we talked not about
present and present perfect, but about two forms of a present perfect,
where present announces a probable present perfect, like saying “she plays” to
mean “she has started to play”, some of her playing is already past!